1. Field of the Invention
The present invention relates generally to radio direction finding systems, and more particularly to a radio direction finding apparatus and method for determining the direction of origin of an RF signal having a frequency that varies from an expected frequency, without requiring the use of multichannel receivers or fixed bandwidth scanning.
2. Description of the Background Art
Direction finding is the process of determining the location of origin of an electromagnetic signal transmission. The process therefore involves first detecting the signal and then determining a direction of arrival (DOA) angle to the signal origin point. According to one method of accurately determining the origin of the transmitted signal, at least two independent directional readings are obtained at different locations simultaneously, and phase differences between the two readings are measured to determine the DOA. Such a method is referred to as triangulation.
Another method of direction finding is to detect a transmitted signal using a rotating antenna, and determining the received signal strength at each rotational position of the antenna. The position of the antenna which gives the highest received signal strength corresponds to the direction of origin of the signal.
There are two types of direction finding. The first type, known as cooperative direction finding, involves locating the origin of an intended signal transmission, such as an electronic beacon, by pinpointing a broadcast origination location. In this case, the source may be an Emergency Location Transponder (ELT), which is used with the so-called xe2x80x9cblack boxxe2x80x9d required to be carried on aircraft. Additional cooperative sources may be RF tag tracking systems attached to personal property, to individuals, or to devices such as land mines.
Cooperative direction finding is typically implemented by encoding the transmitting signal with a signature such as, for example, an audio frequency xe2x80x9cwhoopxe2x80x9d or periodic sweep over a predetermined audio frequency range. The direction finding receiver uses a priori knowledge of the signature to detect the signal. For example, the receiver can demodulate the received signal and perform a cross-correlation with the expected signature. The output of the cross-correlation process indicates the presence or absence of the desired signal as a function of the amplitude of the cross-correlation output signal. The amplitude also may serve as an indication of the DOA when obtained with a rotating directional antenna.
The second type, known as non-cooperative direction finding, involves locating a source of RF transmission, which may represent a hostile entity, or may represent a source of noise or interference with a radio communication system, which is desired to be eliminated. The present invention can be used with both types of radio direction finding systems; however for purposes of explanation the invention will be described with relation to a cooperative radio direction finding system.
Accurate direction finding becomes more difficult if the quality of the transmitted signal is poor, for instance, if the frequency of the transmitted signal is unstable, varies or drifts over time, or has a constant but unknown frequency bias error.
In the case of a low-quality transmission signal, the direction finding receiver according to the conventional art employed one or more compensatory measures. One such measure is to increase the detection bandwidth of the receiver to increase the probability of detecting the transmitted signal. However, as is well known, increasing the bandwidth of the receiver also increases the probability of receiving noise. Consequently, the overall signal-to-noise ratio (SNR) of such receivers is degraded, which results in a reduced effective range of direction finding.
Another approach has been to maintain the bandwidth of the receiver, but to employ a multichannel receiver to detect the RF transmission source. A multichannel receiver in essence is a number of identical receivers operating in parallel, whose bandwidths each represent a different frequency channel or portion of the RF spectrum of interest. The aggregate of the channels corresponds to the entire RF spectrum of interest. While effective, multichannel receivers are complex and expensive.
A third approach involves scanning the RF bandwidth of interest with a receiver of a fixed IF bandwidth. While this approach theoretically may come close to the performance of a multichannel receiver, in the real world the performance of such a scanning receiver is limited by scan rate limitations.
Of course, such compensatory receiver measures become unnecessary if the transmitter frequency is made very stable. Under such circumstances a very simple receiver may be used to achieve near-optimal SNR performance. However, it is not always possible to ensure a stable transmitter frequency. For example, where the transmitter is a tagging device, its proximity to other objects cannot be controlled. The nature of such objects will cause the frequency of the transmitter oscillator to be xe2x80x9cpulledxe2x80x9d to various extents, and consequently the frequency of the transmitter will change over time. Moreover, because of size and power constraints, it may not be possible to provide such transmitters with the necessary frequency stabilization electronics. Long-term aging of a transmitter in the field will cause the transmitting frequency to shift over time. This frequency shift could place the transmitted signal outside of the bandwidth of the receiver, and thereby disable the direction finding system.
Stable transmitter frequency also cannot be guaranteed where the transmitter source is non-cooperative, i.e., where the source may be a hostile transmitting source or a passive noise source, it is not under the control of the party carrying out direction finding.
The prior art has used Fourier transforms in order to aid in signal identification and direction finding. The Fourier transform may be used to convert an electromagnetic signal from the time domain to the frequency domain. This may be done to aid in detection and characterization of a subject signal. For example, U.S. Pat. No. 5,768,477, incorporated herein by reference, uses an FFT (Fast Fourier Transform) array processor to receive a time series of samples from N channels of a multichannel receiver which receives multiple signals, and to provide a series of complex values for each channel. A computer estimates a covariance matrix from the complex values outputted by the FFT array processor, and the covariance matrix is inputted to a neural network to determine the number of received signals and the angles of arrival of each of the multiple signals.
As noted, the ""477 patent requires multiple receivers and is therefore complex and expensive. There thus remains a need in the art for improvement in radio direction finding systems.
A radio direction finding system for determining a direction of origin of a transmitted RF signal is provided, wherein an RF receiver receives the entire RF band of interest. The detected RF signal is subjected to an FFT process, wherein each frequency bin of the FFT process serves as an IF filter for the received RF signal and acts as a separate channel. Each FFT bin is independently processed to demodulate an encoded signature from the RF signal and to provide an FFT baseband. The FFT baseband samples are compared to determine the channel having the greatest SNR with respect to the demodulated signature frequency as a signal level.
In particular, the invention provides a radio direction finding system for determining a direction of origin of a source of a transmitted radio frequency (RF) signal, including an antenna capable of receiving the RF signal, an RF receiver coupled to the antenna system, the RF receiver having a bandwidth for receiving an entire RF band of interest in which the transmitted RF signal may lie, and outputting a detected signal, an analog-to-digital (A/D) converter coupled to the RF receiver and digitizing the detected signal to provide a series of RF samples, a Fast Fourier Transform (FFT) processor coupled to the A/D converter and performing a Fourier transform on the series of RF samples, the FFT processor producing a plurality of component frequency approximations representing frequency components of the detected signal into a plurality of FFT bins, each having a predetermined frequency size, a plurality of demodulators, each receiving the contents of a corresponding FFT bin, and subjecting the contents to a demodulation process whereby an FFT baseband sample is produced for each of the plurality of FFT bins, and a signal-to-noise ratio (SNR) processor that determines a maximum SNR from among the plurality of FFT baseband samples, and outputs an FFT baseband signal whose SNR is determined to be greatest, wherein the outputted FFT baseband signal having a greatest SNR is used as a direction finding metric for determining a direction of arrival (DOA) angle of the RF signal.
A method of radio direction finding is also disclosed.
The above and other features and advantages of the present invention will be further understood from the following description of the preferred embodiments thereof, taken in conjunction with the accompanying drawings.